The `current' object is a central repository for global information.
It provides other objects with names for the current model, contour,
image, etc.

Initially I put this in to minimize the number of global variables that
would need to be hard-coded into object classes. At the moment, the
`current' object is still treated as a global variable; a better
approach might be to specify it when creating new objects which need to
reference other global objects.